For example, in fields such as home appliances, industries, and automobiles, a motor driving device for rotational speed control of a fan, a pump, and a compressor, torque control of electric power steering, and positioning control of a transport machine, an elevator, and the like are used. In the motor driving device, a permanent magnet type synchronous electric motor (hereinafter, referred to as “PM motor”), which is a small-sized and highly efficient alternating current (AC) electric motor, is widely used. However, information on a magnetic pole position of a rotor of the PM motor for controlling driving of the PM motor is required, so that a position sensor such as a resolver or a hall IC is indispensable. Recently, without using the position sensor, sensorless control for controlling rotation speed and torque control of the PM motor is spread.
By realizing the sensorless control, it is possible to reduce a cost of the position sensor (cost of the sensor itself, cost of wiring of the sensor, and cost of attaching and adjusting the sensor) and as the sensor becomes unnecessary, the device can be miniaturized and can be used in a bad environment.
Currently, the sensorless control of the PM motor adopts a method of directly detecting the induced voltage (speed electromotive voltage) generated by rotation of the rotor and driving the PM motor by position information of the rotor or a position estimation technology of estimation-operating the rotor position from a mathematical model of the PM motor.
The method is based on a method using the speed electromotive voltage in principle, so that it is difficult to be applied to a region where the speed electromotive voltage becomes small, such as a stop range, a low speed region, or the like. Therefore, these technologies are mainly applied to the speed range above a medium to high speed range and open loop control such as V/F constant control is used in the low speed region. In a case of the open loop control, since the torque generated by the motor cannot be freely controlled, controllability in the low speed region is poor and efficiency also deteriorates.
Regarding to the problem, a method of obtaining the rotor position information from the low speed region is proposed.
In PTL 1, a pulse voltage is applied to two phases of the PM motor among three phases to detect the open voltage of the remaining one phase not energized, so that position information is obtained from the voltage. Since the electromotive voltage of the open phase is generated according to the rotor position of the PM motor, the electromotive voltage can be used for estimation of the rotor position. The electromotive voltage is a voltage generated by slight inductance change in the motor by a relationship between a permanent magnet-magnetic flux attached to the rotor of the PM motor and a current energized by the pulse voltage, so that the electromotive voltage can be observed in a stop state. This voltage is called “magnetic saturation electromotive voltage”.
In addition, in this method, since the electromotive voltage of the not-energized phase (open phase) is observed, among the three phases, a 120-degree energization driving for selecting and energizing the two phases is indispensable. For position-sensorless driving, it is required that these energized phases are switched according to the position of the rotor. For switching these energized phases, “magnetic saturation electromotive voltage” generated in the open phase is used.
The magnetic saturation electromotive voltage is changed to be monotonously increased or decreased according to the position of the rotor. In PTL 1, “threshold value” is provided in the open phase electromotive voltage. When the magnetic saturation electromotive voltage reaches the threshold value, position-sensorless control is performed by switching to the next energization phase. At that time, “threshold value” is a very important setting factor. The threshold value has subtle variations for each of phases or phase windings of the motor and it is necessary to set threshold value appropriately. A method of automatically executing adjustment work for each of motors is described in PTL 2.
In PTL 2, with respect to the method described in PTL 1, by executing an automatic threshold value adjustment routine in advance, it becomes unnecessary to manually adjust the threshold value by an operator and start-up work of a system is saved.
Although the 120-degree energization driving is premised in these related arts, a sinusoidal wave driving method is already reported. In PTLs 3 and 4, by observing a connection point potential (referred to as neutral point potential) of a Y-connected three-phase windings using the three-phase stator windings Y-connected as the PM motor, the rotor position is estimated.
Since it is unnecessary to observe the open phase in the same manner as in PTL 1, it is possible to energize three phases at the same time and to drive the PM motor with an ideal sinusoidal current. However, detection of the neutral point potential is indispensable.
In PTL 3, a method of inserting a voltage pulse for observing the neutral point potential is described. In addition, in PTL 4, it is possible to immediately estimation-operate the rotor position by using a voltage applied to the inverter for driving the PM motor and observing the neutral point potential by interlocking with a PWM pulse when a pulse width is modulated.